Mapping the global distribution of C4 vegetation using observations and optimality theory

Plants with the C-4 photosynthesis pathway typically respond to climate change differently from more common C-3-type plants, due to their distinct anatomical and biochemical characteristics. These different responses are expected to drive changes in global C-4 and C-3 vegetation distributions. However, current C-4 vegetation distribution models may not predict this response as they do not capture multiple interacting factors and often lack observational constraints. Here, we used global observations of plant photosynthetic pathways, satellite remote sensing, and photosynthetic optimality theory to produce an observation-constrained global map of C-4 vegetation. We find that global C-4 vegetation coverage decreased from 17.7% to 17.1% of the land surface during 2001 to 2019. This was the net result of a reduction in C-4 natural grass cover due to elevated CO2 favoring C-3-type photosynthesis, and an increase in C-4 crop cover, mainly from corn (maize) expansion. Using an emergent constraint approach, we estimated that C-4 vegetation contributed 19.5% of global photosynthetic carbon assimilation, a value within the range of previous estimates (18-23%) but higher than the ensemble mean of dynamic global vegetation models (14 +/- 13%; mean +/- one standard deviation). Our study sheds insight on the critical and underappreciated role of C-4 plants in the contemporary global carbon cycle.